Weeds represent a heterogeneous group of plants, usually defined by no commercial or aesthetic value. Important allergenic weeds belong to the plant families Asteraceae, Amaranthaceae, Urticaceae, Euphorbiaceae, and Plantaginaceae. Major allergens from ragweed, mugwort, feverfew, pellitory, goosefoot, Russian thistle, plantain, and Mercurialis pollen have been characterized to varying degrees. Four major families of proteins seem to be the major cause of allergic reactions to weed pollen: the ragweed Amb a 1 family of pectate lyases; the defensin-like Art v 1 family from mugwort, feverfew, and probably also from sunflower; the Ole e 1-like allergens Pla l 1 from plantain and Che a 1 from goosefoot; and the nonspecific lipid transfer proteins Par j 1 and Par j 2 from pellitory. As described for other pollens, weed pollen also contains the panallergens profilin and calcium-binding proteins, which are responsible for extensive cross-reactivity among pollen-sensitized patients.